Setting indication device for sextants



March 1949. w. w. SHEFFIELD SETTING INDICATION DEVICE FOR SEXTANTS FiledApril 16, 1945 IN V EN TOR.

Patented Mar. 8, 1949 SETTING INDICATION DEVICE FOR SEXTANTS Walter W.Sheffield, Atlanta, Ga.

Application April 16, 1945, Serial No. 588,476

2 Claims.

This invention relates to sextants and more especially to an attachmentthereto, one of the objects being to provide a structure which, whenapplied to a sextant, permits the quick adjustment of the index arm toindicate a selected graduation on the arc of the instrument after which,in accordance with the invention, the index arm can be advanced orretracted relative to the graduated arc, there being an indicatingmechanism associated with said attachment by which an accurate readingcan be had of the minutes and seconds of the measured angle.

With the foregoing and other objects in view which will appear as thedescription proceeds, the invention consists of certain novel details ofconstruction and combinations of parts hereinafter more fully describedand pointed out in the claims, it being understood that changes may bemade in the construction and arrangement of parts without departing fromthe spirit of the invention as claimed.

In the accompanying drawing, the preferred form of the invention hasbeen shown.

In said drawings:

Figure 1 is a plan view of portions of the index arm and of an armcarrying the indicating mechanism, positioned on a portion of the arc ofthe sextant.

Figure 2 is a plan view of said arms, showing the case in which theindicating mechanism is mounted in section, the mechanism therein beingshown in plan.

Figure 3 is a horizontal section through the case taken below the levelof the indicator mechanism on the line 3-3, Figure 4.

Figure 4 is a section on line 4-4, Figure 2.

Referring to the figures by characters of reference, I designates theindex arm of a sextant mounted for swinging movement relative to an arm2, that constitutes a carrying arm for an indicating mechanism to behereinafter described, both arms overlying the graduated are 3 of theinstrument. The index arm I has an opening 4 through which a portion ofthe graduated arc can be viewed and adjacent to the center of the outerwall of this opening 4 is an index or pointer 5'.

A sleeve 5 having a screw-threaded opening therethrough is pivotallyconnected as at 6 to the index arm, the axis of movement of the sleevebeing in line with the periphery of the arc 3. This sleeve is engaged byan adjusting screw 1 which is rotatably mounted in a case 8 carried bythe carrying arm 2. By means of a knob 9 or the like the screw can berotated back and forth for the purpose of feeding the sleeve 5 away fromor toward case 8, thereby to cause the arm I to move away from or towardthe arm 2.

Screw l carries a gear H] which can form one of a chain of gears wherebymotion is transmitted from screw 1 to the shaft 1 l of an indicatingmechanism, this constituting a driving shaft for the mechanism, andconstituting also a delicate-adjustment shaft, as will be hereinafterdescribed. This mechanism includes a dial l2 for indicating seconds, theperiphery of said dial being graduated to indicate seconds, which aresuitably designated. The indicator also includes two minute dials l3 andM, the periphery of one of which is divided into 10 spaces designatedconsecutively from 0 to 9. The other dial, I4, has its periphery dividedinto groups each consisting of 6 consecutively designated spacesnumbered from 1 to 6. Thus one rotation of the dial l2 will indicate upto 60 seconds while the dials l3 and I4 can be automatically operated bythe rotation of dial I2 to indicate consecutively the minutes from 1 to60.

it is well known in the art and, therefore, need not be described orshown in detail.

Shaft II has a knob I5 or the like for the purpose hereinafter explainedand an intermediate or idle gear I0 is included in the train of gears inso that the two knobs 9 and [5 will rotate in the same direction. Theperiphery of the arc 3 is formed with a continuous groove l6 and thatportion of the periphery below the groove is provided with teeth I Ieach corresponding with one degree. In other words, if there are 60graduations on the arc 3 indicating 60 degrees, there would be 60 teethin the same arc, one for each of the indicated degrees.

The case 3 has a pin I8 journaled therein and there is an opening l9provided in the bottom of the case adjacent to this pin. One end of thepin carries a toothed jaw 20 positioned to engage with some of the teethI! and an arm 2| is also extended from thepin. This arm is located inthe case 8 and is held normally pressed in one direction by spring 22. Aplunger 23 is slidably mounted in one wall of the case, the inner end ofthe plunger being in engagement with arm 2| while the outer end has afinger piece on knob 24 whereby the plunger can be shiftedlongitudinally to actuate arm 2! against the spring 22 and disengage thetoothed jaw 20 from the teeth IT.

The carrying arm 2 is held to the are 3 by means of a hooked tongue 25extending from the No special mechanism is provided for effecting thisoperation of the indicating dials as arm 2 and into groove l6, withinwhich it is adapted to slide when said arm is adjusted.

It is to be understood, of course, that the top of the case 8 isprovided with an opening 26 through which the peripheries of the dialsl2, l3, and M are exposed to permit convenient reading of the indicator.

As is well known a circle is divided into 350 degrees, each degree beingdivided into sixty parts designated by the geometrical term minutes arc,and each minute is likewise divided into sixty parts designated asseconds of arc. There are therefore 1,296,000 seconds to a circle andthe present invention provides a means for so dividing a circle intosingle seconds, or 1,296,000 parts, these small divisions being readfrom the dials 12-43-44. By means of the present invention each of thegraduations of are 3 can be divided into 3600 parts.

It is essential that at the beginning of the operation the dials |2-l3l4all read zero and that the index pointer 5 shall exactly coincide withone of the graduations of are 3. The sextant is so constructed with atoothed periphery to are 3, that with the dials l2-I'3--I4 reading zeroand the carrying arm 2 clamped, the pointer 5 will so coincide with oneof the graduations on are 3. It is further essential that sixty turns ofdial it shall be required to move the index pointer 5 one graduation (ordegree) on the scale of are 3. Therefore it is essential that thecombination of the gear train l and the screw 1 shall be such as toprovide a movement of the index arm I, and pointer 5, through onegraduation (degree) of the scale of are 3 by sixty turns of the dial l2.Consequently, one complete turn of dial 12 will move the index pointeronly of one graduation (degree) on the scale of are 3 and at this pointdial 13 will register the figure 1, this 1 representing /60 of a degree,or one minute of arc. Likewise, 21 turns of the dial I2 will move theindex pointer 5, of a graduation of scale of are 3 and dials l3 and Mwill now read 21, this 21 being of a graduation (degree) on scale of are3, or 21 minutes of arc, and so on. Any fraction of a turn of dial 12will be shown by the graduations along the periphery of said dial l2,and as there are sixty graduations the reading of dial IE will indicatesixtieths of the graduations of dial [3, which, of course, will beseconds of are. In reading the angle measured the degree reading istaken directly from the scale of are 3, while the minute and secondreadings are taken from the dials i2l3-M. In the drawings the reading onthe are 3 is 21 degrees, while the dials read 21 minutes, seconds. Inother words the reading 21 degrees, 21 minutes, 20 seconds in theillustration shows that the arm 2 was clamped at a position wherebyindex arm 1 (as shown by pointer 5) was exactly at a reading of 2| onthe scale of are 3, and that the screw 1 was then turned a proper numberof turns so as to rotate dial l2 twenty-one times as indicated by thereading 21 on the dials l3 and IA, and that the dial l2 was furtherrotated of a turn as indicated by the reading 20 on dial 12. As theratio of movement is 60 turns of dial l2 to one graduation (degree) onscale of are 3, the index pointer 5 will now be advanced to a pointbetween 21 degrees and 22 degrees on the scale of are 3, the exactamount of movement of index arm l and pointer 5 being 21 minutes and 20seconds of are as shown by the dials l2l3l4.

In operating a sextant the sun, or any other heavenly body whosealtitude we are measuring is brought down to a point where it appears tojust touch the horizon. This is done by a combination of mirrors and asmall telescopic eyepiece. Navigators call this point of apparentcontact kissing the horizon. One of the mirrors is attached to the indexarm and by the movement of this arm along the arc of the sextant, andthe corresponding movement of the index mirror the angle is measured. Inthe final setting of the sextant, the moment of kissing the horizon is avery delicate operation, the accuracy of which may mean a difference ofmany miles in working up a position at sea. Although the knob 9transmlts a slow motion to the index arm l by means of the screw 1, itimparts a rather fast movement of the dial l2. Knob I5 is not actuallynecessary in the operation of the invention, biE is added as a meanswhereby a setting of extreme delicacy may be effected at the finaladjustment. As the movement of the indicating mechanism may be actuatedby either knob 9 or knob iii, an idle gear has been inserted in the geartrain it to provide rotation in the same direction on the two knobs 9and I5. It would be very inconvenient for these knobs 9 and i5 to rotatein opposite directions. Knob l5 and the extended shaft H are intended toactuate dial l2 only. The dials l3 and M which are mounted on a hollowsleeve or shaft 21 through which shaft H extends, are operatedautomatically by the rotation of dial l2. Shaft H carries dial I2 andits motion can be controlled by knob l5. Resetting the dials is effectedby either knob 9 or knob i5, thus operating the entire mechanism so thatwhen the dials all read zero the index pointer will again exactlycoincide with one of the degree readings on scale of the are 3.

A pointer 21 is provided in Fig. 1 adjacent to dial I2 to facilitate thecorrect reading of the graduations along its periphery.

What is claimed is:

1. In a sextant, an attachment for determining fractional values ofdegrees in minutes and seconds, comprising, in combination with an indexarm and a degree-graduated are along which the index arm is mounted forswinging movement. a carrying arm mounted for swinging movement alongthe arc, an adjusting screw rotatably mounted on said arm, a threadedsleeve pivotally connected to the index arm, the adjusting screw beingthreadable in the sleeve, teeth on the arc, there being one tooth foreach graduation thereof, a spring-pressed plunger mounted on thecarrying arm, a toothed jaw pivotally connected to the carrying arm andoperable by the plunger to engage selected teeth of the arc, whereby toremovably clamp the carrying arm in any of a number of exact positionsof adjust ment corresponding to the degree-graduations of the arc, anindicating mechanism on the carrying arm indicative of minutes andseconds of degrees, a shaft, rotatably mounted on the carrying arm,operatively connected to the indicating mechanism for rotation of theindicating mech anism, means on the shaft permitting manual operationthereof as a delicate-adjustment shaft. and a ratio-gear trainoperatively connecting the adjusting screw and the shaft.

2. In a sextant including an index arm and a degree-graduated are, amanually operated means for providing minute and second readings,comprising a carrying arm swingable on the arc, a manually operatedadjusting screw rotatable on the carrying arm, a threaded connectionbetween the adjusting screw and index arm pivoted to the index arm,teeth on the arc, there being one tooth to each degree-graduation, atoothed jaw pivotally mounted on the carrying arm, a spring carried bythe carrying arm in yielding engagement with the jaw, whereby to retainthe carrying arm normally clamped in engagement with the arc in any of anumber of positions corresponding to degree-graduations on the arc, amanually operated plunger on the carrying arm movable against the jaw todisengage the jaw from the teeth of the are, a rotatabledelicate-adjustment shaft on the carrying arm, an indicating mechanismon the carrying arm driven by The following references are of record inthe file of this patent:

UNITED STATES PATENTS m Number Name Date Re. 22,184 Everitt Sept. 29,1942 1,190,719 Buff July 11, 1916 1,922,976 Otto Aug. 15, 1933

